Pressure drop has been measured for upward single- and two-phase gas–liquid flow across an orifice in a vertical pipe. A conductance probe provided average void fraction upstream of the orifice. Six orifices with different apertures/thickness were mounted in turn in a 34 mm diameter transparent acrylic resin pipe. Gas and liquid superficial velocities of 0–4 m/s and 0.3–0.91 m/s, respectively, were studied. For single-phase flow, pressure drop, expressed as an Euler number, was seen to be independent of Reynolds number in turbulent region. The Euler number increased with decreasing the open area ratio/orifice thickness and increasing velocity. The pressure drop was well predicted by the correlation of Idel'chik et al. (1994, Handbook of Hydraulic Resistances, 3rd ed., CRC Press, Boca, Raton, FL.), which uses a form of Euler number. The corresponding two-phase flow pressure drop depends on the flow pattern. Decreasing open area ratio/orifice thickness increased the pressure drop. For a given liquid superficial velocity, the pressure drop increases with gas superficial velocity except for low open area ratio where this increase is followed by a decrease beyond a critical superficial gas velocity for the high liquid superficial velocities. Relevant correlations were assessed using the present data via a systematic statistical approach. The two-phase multiplier equations of Morris (1985, “Two-Phase Pressure Drop Across Valves and Orifice Plates,” European Two Phase Flow Group Meeting, Marchwood Engineering Laboratories, Southampton, UK.) and Simpson et al. (1983, “Two-Phase Flow Through Gate Valves and Orifice Plates,” International Conference on Physical Modelling of Multiphase Flow, Coventry, UK.) are the most reliable ones.